1. Field of the Invention
The present invention relates to a reflective-type color electronic paper display device to achieve color display by using an electrophoretic display device (EPD).
The present application claims priority of Japanese Patent Application No. 2005-223514 filed on Aug. 1, 2005, which is hereby incorporated by reference.
2. Description of the Related Art
In recent years, research and development of an electronic paper display to be used as a display device which enables an action of “reading” without being subject to stresses, such as electronic books and electronic newspapers, is progressing. Requirements of an electronic paper display device are that it is thin, lightweight, durable (hard to crack), and easy to view (read) the same contents as printed ones. It is also required that, as the color displayed contents increases, such the electronic paper display also can display contents in colors. As the color electronic paper display device that can satisfy these various requirements, a reflective-type color display device requiring no backlight and being able to reduce power consumption can be suitably used.
As such the reflective-type color display device as described above, a reflective-type liquid crystal display device disclosed in, for example, Non-patent Reference 1 (Reflective Liquid Crystal Display, Wiley, SID, pages 4 to 13) is known. However, the disclosed reflective-type liquid crystal display has a problem in that the liquid crystal display device exhibits too narrow directivity of light, that is, due to the use of a metal electrode having a flat surface as a reflective plane, its displayed contents look bright in a limited direction determined by a direction of incident light, however, the displayed contents look dark in a direction other than the specified direction. Additionally, the disclosed reflective-type liquid crystal display device has another problem in that, due to its use of a polarization mode of light, chromatic dispersion of light is made wide and a white color cannot look like white and that, due to its use of a polarizer, the efficiency of exploiting light is low, causing a white color to become dark; in other words, the problem is that perfect paper-white display required in electronic paper is difficult.
On the other hand, as the reflective display device without polarizer, an EPD that uses electrophoretic elements is known. In the following, the EPD that uses microcapsule-type electrophoretic elements is described by referring to FIG. 4.
FIG. 4 is a diagram schematically showing an example of cross-sectional configurations of a conventional monochromatic EPD active matrix display device. As shown in FIG. 4, the monochromatic EPD active matrix display device includes a facing substrate 1, an EPD film 2 which is a film-like electrophoretic display device, and a TFT (Thin Film Transistor) glass substrate 3.
The facing substrate 1 is so configured that a facing electrode 12 made of a transparent conductive film is formed on an inner surface of a transparent plastic substrate 11 made of, for example, polyethylene terephthalate (PET) or a like. The facing substrate 1 may be made of a glass substrate instead of the PET substrate 11.
The EPD film 2 is formed so as to be film-shaped and includes microcapsules 13 which spread all inside of the EPD film 12 and a binder 14 made of a polymer filled among the microcapsules 13 with the purpose of binding these microcapsules 13. Each of the microcapsules 13 attains the size of about 40 μm and a solvent 15 made of isopropyl alcohol (IPA) or a like is hermetically encapsulated inside each of the microcapsules 13. In the solvent 15, white particles 16 each having a nano-level size and being a titanium oxide white pigment and black particles 17 each also having a nano-level size and being a carbon black pigment are floating in a dispersed manner. Each of the white particles 16 has a negatively (−) charged polarity and each of the black particles 17 has a positively (+) charged polarity.
The TFT glass substrate 3 has a four-layer structure. In the first layer nearest to the EPD film 2 is formed a plurality of pixel electrodes P1.1, P2.1, P3.1, . . . . The second and third layers are made up of insulating layers each containing a plurality of TFTs T1.1, T2.1, T3.1, . . . with each TFT corresponding to any one of pixel electrodes P1.1, P2.1, P3.1, . . . in a one-to-one relationship. In the second layer are formed drains (D) and sources (S) each corresponding to any one of the TFTs T1.1, T2.1, T3.1, . . . in a one-to-one relationship. In the third layer are formed gates (G) each corresponding to any one of the TFTs T1.1, T2.1, T3.1, . . . in a one-to-one relationship. The source (S) of each of the TFTs is connected to a corresponding pixel electrode P1.1, P2.1, P3.1, . . . . The fourth layer serving as the lowest layer is a base-body layer made of glass which is formed so as to support the first to three layers in an integrated manner.
In FIG. 4, when a plus (+) voltage is applied to the pixel electrodes P1.1 and P2.1 through the TFTs T1.1 and T2.1, respectively, since the white particles 16 contained in each of the respective microcapsules 13 are attracted and gathered, in relatively higher amounts, toward the pixel electrodes P1.1 and P2.1 and black particles 17 contained in each of the respective microcapsules 13 are attracted and gathered, in relatively higher amounts, toward the facing electrode 12 and, when a minus (−) voltage is applied to the pixel electrode P3.1 through the TFT T3.1, since the black particles 17 contained in each of the respective microcapsules 13 are attracted and gathered, in relatively higher amounts, toward the pixel electrode P3.1 and the white particles 16 contained in each of the respective microcapsules 13 are attached and gathered, in relatively higher amounts, toward the pixel electrode P3.1 and the white particles 16 contained in each of the respective microcapsules 13 are attracted and gathered, in relatively higher amounts, toward the facing electrode 12. For this effect, an image made up of white and black is displayed on the facing electrode 12 side. Thus, in the EPD active matrix display shown in FIG. 4, by applying either a plus (+) voltage or a minus (−) voltage to the pixel electrodes, the white and black image can be displayed on the facing electrode 12 side.
FIG. 5 is a diagram schematically showing an example of cross-sectional configurations of a conventional color EPD active matrix display device. As shown in FIG. 5, the color EPD active matrix display device includes a CF (Color Filter) glass facing substrate 4 having color filters, a film-like EPD film 2, and a TFT glass substrate 3 which has TFTs T1.1, T2.1, T3.1, . . . . Out of these components, configurations of the EPD film 2 and TFT glass substrate 3 are the same as employed in the case of the monochromatic EPD active matrix display shown in FIG. 4 and their detailed descriptions are omitted accordingly.
The CF glass facing substrate 4 is made up of a colored layer 22 having red (R), green (G), and blue (B) resists formed sequentially on the inner surface of a transparent glass substrate 21 and a facing electrode 23 made up of a transparent conductive film. In this case, for example, the light transmitted through the Green (G) colored layer 22 is reflected by the respectively microcapsules 13, which are placed on the pixel electrode P3.1, being put in a state of displaying white, and then travels into a person's eye through the Green colored layer 22, Thus, the green color is recognized.
In the color display having the structure shown in FIG. 5, in order to exactly control each color, it is necessary to perform precise positioning between the color filter for each color making up the colored layer 22 in the CF glass facing substrate 4 and the TFT corresponding to each color in the TFT glass substrate 3, with accuracy of, for example, 5 μm or so.
In the CF glass facing substrate 4, since the glass substrate 21 is used which can provide a strong structure, precise positioning between the CF glass facing substrate 4 and the TFT glass substrate 3 is feasible. However, if the glass substrate 21 is to be replaced with a plastic substrate for reduction of costs, it is impossible to maintain precise positioning between the color filter made of a soft plastic which can deform and the TFT glass substrate 3 with accuracy of 5 μm or so.
FIG. 6 is a diagram showing an example of a cross-sectional configuration of a conventional color EPD active matrix display device in which a film-like color filter substrate is employed. As shown in FIG. 6, the color EPD active matrix display device includes a CF (color filter) plastic facing substrate 5 having color filters, a protecting layer 6 having an adhesive layer 26, a transparent plastic layer 27, a facing electrode 28, an EPD film 2 having electrophoretic elements (microcapsules and binder), and a TFT glass substrate 3 having TFTs T1.1, T2.1, T3.1, . . . . Out of these components, configurations of the EPD film 2 and the TFT glass substrate 3 are the same functions as employed in the case of the monochromatic EPD active matrix display shown in FIG. 4 and their detailed descriptions are omitted accordingly.
The CF plastic facing substrate 5 having a film-like structure is made up of a colored layer 25 in which color filters made up of resists each being for red (R), green (G), and blue (B) are arranged sequentially on the inner surface of a transparent substrate (PET substrate) 24. The protecting layer 6 is made up of the adhesive layer 26 with which the CF plastic facing substrate 5 is laminated, the transparent plastic substrate 27 serving as a supporting substrate, and the facing electrode 28 made of a transparent conductive film.
The color EPD active matrix display shown in FIG. 6 is assembled by overlaying the CF plastic facing substrate 5 on the EPD film 2, however, it is difficult to laminate the CF plastic facing substrate 5 made of a plastic resin directly on the EPD film 2 with precise and direct positioning between the film-like CF plastic facing substrate 5 and the EPD film 2 and, therefore, a method is conventionally adopted in which the CF plastic facing substrate 5 is laminated, in advance, with the protecting layer 6 for reinforcement. To solve this problem, the protecting layer 6 requires the adhesive layer 26 with which the CF plastic facing substrate 5 is to be laminated and, in order to make the CF plastic facing substrate 5 have the same mechanical strength as can be obtained if the glass facing substrate is adopted, the formation of the plastic substrate 27 having a sufficient thickness to serve as the supporting layer in the protecting layer 6 is needed. To achieve this, it is necessary that the protecting layer 6 has a thickness of 50 μm to 100 μm.
In FIG. 6, an example of displaying red (R) by the color EPD active matrix display device having such structures as described above is shown. In the case of the structure shown in FIG. 6, since the protecting layer 6 is thick, the distance between each of the microcapsules 13 and colored layer 25 in the CF plastic facing substrate 5 increases. For this reason, all the incident light through the Red colored layer 25 is been reflected by the microcapsules 13 in a state of displaying white, and then do not go out again through the Red colored layer 25, but a part of the light passes through the Green colored layer, as a result, causing great attenuation and enabling almost no transmission of the light.
If it is now assumed that a pixel element for a single color is 80 μm in width and 240 μm in length and a color pixel for three colors of red (R), green (G), and blue (B) being 240 μm in width and 240 μm in length, which is obtained by arranging the above pixel element, is to be formed, in the case of the Red colored layer, a boundary region of 20 μm between the Red and Green colored layers and also a boundary region of 20 μm between the Blue and Red colored layers are regions that cannot be effectively used and a width of the region of the color pixel that can be effectively used is 80 μm−20 μm (R-G)−20 μm (R-B)=40 μm and, as a result, transmission factor of each of the RGB colors through the color filter is ½ of the entire. Therefore, reflection factor of white light, in the case of monochromatic display, is 40% in comparison with the reflection factor of a standard white plate, while transmission factor of each of the RGB color light components through color filters is ½ and, as a result, in the case of color display, the rate of light components, out of all incident light, that can be effectively used is 40%×½×½=10%, which causes an extremely dark display.
As described above, the conventional color electronic paper display device that adopts the film-like color filter substrate as shown in FIG. 6 has the following problems.
(1) It is necessary that the CF plastic facing substrate 5 and the TFT glass substrate are put in precise and fine alignment. However, it is difficult to put the CF plastic facing substrate 5 made of a soft plastic resin and the TFT glass substrate 3 made of glass in precise alignment with accuracy of 5 μm or so. As a result, the thick protecting layer 6 has to be formed to reinforce the CF plastic facing substrate 5 additionally.
(2) Since the thickness of the protecting layer 6 is increase, the distance between the colored layer 25 and each of the microcapsules 13 increases and a phenomenon occurs in which all incident light from, for example, the Red colored layer cannot go out through the Red colored layer and part of the light components goes out through the Green and Blue colored layers, thus causing extreme low reflection factor of color light in comparison with reflection factor of white light.
Moreover, a conventional liquid crystal display device, as a display device related to the above color electronic paper display device, is disclosed in Patent Reference 1 (Japanese Patent Application Laid-open No. 2003-177429) in which a liquid crystal is driven by an electric field, which is in parallel to a substrate, generated by forming a color filter layer and a liquid crystal layer on a TFT with a liquid crystal filter layer overlain on the color layer and by forming a transparent facing substrate on the liquid crystal layer.
Also, a charged-state detecting apparatus is disclosed in Patent Reference 2 (Japanese Patent Application Laid-open No. 2004-294273) which has an electronic paper display device to display at least one portion in a charged state out of portions in a non-charged state, positively charged state, or negatively charged state of a substance by a color being different from the color that shows a portion in other charged state.
Furthermore, a liquid crystal display device is disclosed in Patent Reference 3 (Japanese Patent Application Laid-open No. Hei 8-9811) which has a two-layer liquid crystal in which a Guest Host (GH)-type Polymer Dispersed Liquid Crystal (PDLC) layer of a first substrate is configured so as to face the GH-type PDLC layer of a second substrate.